Collision course

01 Jun 2014 The Navigator

There are two clear and distinct purposes for which radar is used; navigation and collision avoidance. While there are some fundamentals of radar use that are the same for both tasks, the ideal radar set-up for the two tasks is quite different. In fact, if there are two radars available, it may well be advisable to use one radar for long-range and one for close-range detection, or one for navigation and one for collision avoidance

Although there are clear best practices on setting up a radar for collision avoidance purposes, the tools you use and the way you manage the display can be based on personal preference or local conditions. For example, the situation can be very different if you are in a crowded waterway from when a vessel is on the open sea. Here, members of The Nautical Institute’s Seagoing Correspondence Group (SGCG) offer advice on their own preferred ways of managing radar for collision avoidance.

Radar can help with collision avoidance in many ways. The simplest, perhaps, is to lay an electronic bearing line (EBL) across a suspected target to see if the relative bearing is steady and if a risk of collision exists.

While relative bearings/vectors are highly reliable and accurate, true vectors or decision support tools that rely on inputs, such as speed, position, or gyro data, could be inaccurate if the input data is compromised. Interpreting radar returns is still an art and not an exact science. In other words, assume nothing and always try to check visually, or by an alternative independent system.

It is crucially important to routinely change ranges when detecting and monitoring targets. Use of the longer range settings gives good prior awareness of more distant but relatively large targets, whereas shorter range settings are necessary to detect and discriminate virtually all targets closer to own vessel. The use of offsets is also encouraged. According to one member of the SGCG, “operating with centre offset to maximise look ahead is always preferred for both navigation and collision-avoidance radars.”

Radars have options for automatic tuning of gain, STC (sea clutter), FTC (rain clutter), and AFC (tuning control). However, the automatic settings of these controls will not necessarily give the best performance in all conditions and so manual adjustment can be important. Professional navigators must ensure that they are proficient with the use of these controls and be familiar with them for every ship they sail on. Poor manual tuning is a real hazard, as is failure to recalibrate the radar when conditions change. Best practice is to always check the settings of all controls before assuming a watch.

Using one radar for navigation and the other for collision avoidance is best in some circumstances, while in others, it may be beneficial to use one radar for greater range and the other for close-range detection. It is important, too, to recognise the difference between S-Band (3GHz/10cm) and X-Band (9GHz/3cm) radars, both of which are required on larger vessels. S-Band is generally preferable in adverse conditions, such as fog, rain and heavy seas, while X-Band is noted for good angular discrimination, required in pilotage waters, for example. On ships where the radar displays can be inter-changed, such as on Integrated Bridge System workstations, it is essential to know which radar system (S or X) you are using at any time, and the expected performance parameters for each.

Shipboard radar displays can be configured in a range of orientation modes (head-up, North-up, course-up) each offering benefits and hazards. Head-up display allows easy association with views from the bridge windows or from electronic charts in headup mode, whereas North-up gives easy association with paper charts or electronic charts in North-up mode. These choices may also be affected by the area the ship is in, such as pilotage waters or open ocean, and should always be coordinated with all members of the bridge team. As one experienced pilot advises, “learn how to use a head-up unstabilised display, so that if/ when all the secondary inputs fail, you still have a useful tool.”

Radar and AIS data can only be used safely if there is a good understanding of both true and relative motion. Poor understanding can result in a major collision! Displayed vectors on radar-tracked targets and AIS-acquired targets can be set to be either true or relative. Again, the combination of settings and uses will depend on the individual circumstances. One mate suggested that, “for collision avoidance, the use of relative vectors and true trails is most useful.”

Radar and AIS data can be very effective if used together, either manually or automatically (association). Benefits and weaknesses of operating radar and AIS together include:

  • Two independent ways of detecting targets
  • Two independent estimates of a target’s range, bearing, course and speed
  • Radar detection of targets that do not carry AIS
  • Clear AIS transmissions, almost unaffected by clutter
  • AIS can be ‘seen’, whereas radar detection can be impossible, e.g. behind islands and headlands
  • Radar doesn’t have to rely on external data sources, unlike AIS
  • AIS can indicate changes in course and speed quicker than radar can detect them
  • AIS can often provide more information about a target

Information overload is a serious danger. Modern radars can display a wide range of additional information and symbols, but too much information can lead to confusion and non-detection of targets. Such additional information can come from ARPA, AIS, vectors and vector lengths, target names and information, guard zones, exclusion zones, chart information, etc. As the shipping industry migrates towards greater use of Integrated Bridge Systems (IBS), Integrated Navigation Systems (INS) and eNavigation, the need to manage portrayed information will become ever more essential. Never forget that it is easier to detect targets on a relatively clear display.

Further information about using radar for collision avoidance can be found in the Nautical Institute’s guidebook, Radar and AIS by Dr Andy Norris.